Project Summary The suprachiasmatic nucleus (SCN) is the primary circadian oscillator in the central nervous system, entrained to the day/night cycle via the retinohypothalamic tract. The circadian-timing system has a complex architecture. In addition to the SCN, subsidiary clocks are located in most, if not all, tissues, organs, and cells of the body including brain regions distinct from the SCN. Peripheral clocks directly regulate local rhythms in cellular metabolism and hormone secretion and require daily entraining cues from the SCN for coordinated timing of behavioral, physiologic and metabolic circadian rhythms, a primary requisite for a healthy body and mind. The SCN maintains global circadian synchrony via its connections with autonomic circuits innervating peripheral organs and by its regulation of rhythmic hormone secretion such as adrenal glucocorticoids. Rhythmic corticosterone (CORT) signals induce the rhythmic expression of a diverse array of genes including clock genes. Temporal homeostasis is a complex interplay between central and autonomic neural circuits and hormonal feedback from the adrenal. Changes in circadian function and the accompanying changes in phase have been associated with several human disorders. A reduction in the amplitude of the CORT diurnal rhythm may exert a wide range of effects on metabolism and central nervous system function. Preliminary data demonstrate that alterations in entrainment of the SCN to the day/night cycle produce changes in the diurnal CORT rhythm; as entrainment phase angle is progressively more delayed relative to light offset the amplitude of the diurnal corticosterone rhythm is progressively reduced, up to as much as 50%. Specific Aim 1 uses transcriptional profiles of clock genes to extend preliminary findings and examines potential mechanisms by which altered entrainment to the day/night cycle reduces the amplitude of the diurnal CORT rhythm. Specific Aim 2 describes the neural circuits (that may circumvent the SCN) that send signals to the adrenal. Retinal input to pre-autonomic neurons is identified by anterograde tracing of retinal efferents to the hypothalamus in conjunction with labeling of pre-autonomic neurons in the hypothalamus via transneuronal retrograde tracing using pseudorabies virus injected into the adrenal. Functional experiments target identified pre-autonomic hypothalamic neurons for neurotoxic lesioning to determine effects on adrenal function. Specific Aim 3 utilizes transplantation of adrenals from mice with arrhythmic adrenal oscillators (Per2/Cry1 dKO mice) into adrenalectomized wild type mice with altered entrainment to dissect the functional roles of the SCN and adrenal oscillators, and the L:D cycle on the regulation of the diurnal rhythm of CORT secretion. Understanding how retinal circuits and the central clock regulate peripheral oscillators via autonomic circuits will aid in our ability to better understand and treat altered circadian rhythms.